Plant hormones (phytohormones) are chemicals that regulate plant growth, which are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, moved to other locations, in other functional part of the plant. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Plants, unlike animals, lack glands that produce and secrete hormones. Instead, each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also known as growth factors or growth hormones.
Hormonal action is regulated by the synthesis of a hormone and its accumulation in active form, which is also affected by conjugation and degradation.
Transport also controls regional accumulation of the hormone, but in many cases we know little about how hormones are regulated. Hormones initiate signaling by binding to receptors. Some hormone receptors are transmembrane proteins that undergo a conformational change upon hormone binding, whereas for others the binding of hormone serves as "molecular glue" to facilitate the interaction between two proteins. Downstream signaling often includes protein phosphorylation and proteolysis, and culminates in changes in transcription, ion channel activities and other effects.
Auxins are a family of related compounds that were originally identified as promoters of growth; their name derives from the Greek word auxein, meaning "to grow." Auxin promotes growth and cell elongation but also has critical roles in embryonic pattern formation, promoting and specifying the positions of vascular tissues and leaf and lateral root initiation, and maintaining stem cell populations. Auxin synthesis is tightly regulated and contributes to the auxin gradients that underlie developmental patterning. The most abundant naturally occurring auxin is indole-3-acetic acid (IAA). IAA is chemically similar to the amino acid Trp from which it is synthesized via several different enzymatic pathways.
The regulated transcellular movement of auxin between cells is critical to its action. Auxin is transported into and out of cells by families of auxin influx and efflux carriers, setting up auxin gradients and local auxin maxima or minima. These gradients and maxima/minima are sufficient to trigger morphogenetic events, including leaf initiation and lateral root initiation. Auxin distribution plays a key role in embryonic pattern formation, including specifying the cells that will form the root stem cell population.
Auxin also acts through its interaction with another receptor protein, Auxin Binding Protein1 (ABP1). ABP1 is associated with auxin responses at the plasma membrane, including activation of a proton pump and cell wall acidification, and contributes to auxin-regulated gene expression. At this point, we don’t know how signaling downstream of ABP1 is transduced this is an unfinished chapter in the auxin story.
Cytokinins (CKs) are a family of related compounds that are derived from adenine. CK biosynthesis and catabolism are strongly regulated by hormones and inorganic nutrients. Most plants make multiple CKs that interact with differing specificities with different CK receptors, which may fine-tune CK signaling. CK transport mechanisms are not well understood, but there is evidence that they are translocated from root to shoot.
CKs are perceived by a family of membrane-localized receptors that form part of a two-component system that closely resembles bacterial two-component systems. At the shoot apex, auxin promotes lateral organ initiation, whereas CK maintains the cells in an undifferentiated, proliferating stem cell state. At the root apex, auxin maintains the stem cell population and CK induces differentiation. Experiments have revealed a simple signaling network through which auxin and cytokinin coordinate these activities at the root apical meristem. A high auxin level in the stem cells (as a consequence in part of polar auxin transport to the root tip) promotes cell division and directly represses the expression of CK biosynthetic enzymes.
Antagonistic effects of auxin and CK similarly control the outgrowth of branches in the shoot and root.
A localized auxin maximum is sufficient to initiate the outgrowth of a lateral root, whereas CK represses lateral root initiation. In the shoot, axillary bud meristems are formed in association with a leaf. The outgrowth of these buds is controlled in part by auxin; early experiments showed that the decapitation of a plant allowed them to grow out, whereas decapitation followed by auxin application to the cut site prevented them from growing out (referred to as the apical dominance effect). Studies in intact plants showed that CK antagonizes auxin’s effects, promoting bud outgrowth, and that auxin derived from the apex represses cytokinin biosynthesis at the bud. A third hormone that inhibits bud outgrowth was later identified as a strigolactone.
CKs also have important roles in controlling plant nutrient uptake and allocation, nitrogen-fixing root nodule development, root and shoot architecture and seed yields, and leaf senescence. Recently, drought-tolerant plants were produced by introduction of a drought-induced CK biosynthesis gene, providing a new and exciting approach for enhancing plant growth under suboptimal conditions.
Strigolactones are exuded by plant roots into the soil and are recognized and responded to by mutualistic mycorrhizal fungi.
Parasitic plants of the Striga genus (for which the compound is named) eavesdrop on this communication and respond to the presence of strigolactones by germinating, penetrating the roots of the host, and withdrawing nutrients. In many parts of the world, parasitic Striga are a major cause of reduced crop yields. Recently, strigolactones were found to participate in the control of shoot branching, demonstrating that these compounds serve as intraorganismal signaling hormones as well as inteorganismal signals.
Auxin transported from shoot to root induces the synthesis of strigolactones, which are ultimately translocated into the shoot, where they interfere with bud outgrowth. Decapitation of the apex reduces auxin flow into the root and strigolactone synthesis, facilitating bud outgrowth, and mutant plants that don’t make strigolactones produce extra branches. In some plants, strigolactone synthesis is induced upon nutrient limitation. By promoting root growth and mycorrhizal symbiosis while limiting shoot growth, strigolactones help optimize growth patterns for nutrient acquisition under nutrientlimited conditions. The full details of how this newly identified hormone is synthesized and functions are still being elucidated.
Gibberellins are a family of compounds, only some of which have biological activities in plants; gibberellic acid (GA3) is the most active and the most well characterized. GA accumulation is tightly regulated by the control of key enzymes in its synthesis and degradation. Gibberellins were first identified biochemically through investigations of the strange effect that a fungal pathogen, Gibberella fujikuroi, has on its host plant. Infected plants grow extremely tall, aren’t able to support themselves, and fall over to root. Because of this odd effect, the disease caused by the fungus was called bakanae, which translates to foolish seedling.
Later, gibberellins were identified as endogenous plant hormones that control diverse aspects of plant development. The biosynthetic pathway for gibberellins was deduced by analyzing GA-deficient mutants. Early steps occur in plastids, but subsequent steps occur in the endoplasmic reticulum and cytoplasm.
GA dwarfism is an extremely valuable agricultural trait, especially in grasses, because shorter, sturdier stems are better able to support the large, heavy seeds that well-fertilized crop plants produce; dwarf varieties produce higher grain yields due to increased resource allocation into the seed as well as fewer plants falling over and rotting. In the green revolution of the 1960s, crop yields doubled as a result of increased fertilizer use in combination with semidwarf varieties. Although their value in grain production was recognized more than 50 years ago, the gene products of the green revolution semidwarf genes were only recently identified. The rice (Oryza sativa) green revolution gene semi-dwarf1 encodes an enzyme in the GA biosynthetic pathway, GA 20-oxidase.
Gibberellins have several other functions in plant growth and development, only some of which are understood at the molecular level. Many if not all of these are mediated through the interaction of gibberellins with the DELLA proteins.
For example, gibberellins promote flowering, by destabilizing the DELLA proteins that interfere with the transcription of genes that promote flowering, promote root growth by destabilizing the growth inhibitory effects of DELLA proteins in the root, and promote seed germination by inactivation of DELLA proteins that promote the action of the dormancy-promoting hormone ABA. It appears that gibberellins do much more than control stem elongation rather, they may be a key node in many of the cross-regulatory interactions among the plant hormones.
Brassinolide and its related plant steroid hormones are collectively called brassinosteroids (BRs) to reflect their initial characterization in Brassica, although they are present in all plants and some algal species as well. BRs are synthesized from campesterol, a sterol. The identity of many of the biosynthetic genes was determined from the study of dwarf mutants that don’t produce BRs.
BRs are perceived by membrane-localized Leu-rich repeat receptor-like kinases. Hormone binding initiates a protein kinase cascade, ultimately leading to changes in gene expression. BRs participate in diverse processes, including vascular and reproductive development, control of plant architecture, light responses, and stress responses.
BRs promote cell elongation. BR-induced genes include genes that loosen cell walls to permit cell expansion by the internal turgor pressure within plant cells. In part because of their growth-promoting effects, some BR-overproducing (or Rhypersensitive) plants can produce higher yields. On the other hand, the uzu mutant of barley (Hordeum vulgare) has a mutation in the BR receptor that makes it semidwarfed, resistant to blowing over in the wind, and also higher yielding. Thus, understanding the role of BR in cell elongation opens up several opportunities for crop yield improvements.
Ethylene promotes ripening in many fruits. Fruit ripening is a complex process that includes changes in color, flavor through the breakdown of starches into sugars, and texture through changes in cell wall structures. The important role of ethylene is vividly seen in mutants affected in ethylene production or response. In ethylene-ripened fruits (called climacteric fruits), such as tomato (Solanum lycopersicum), a rapid increase in ethylene synthesis promotes a rapid ripening response across the entire fruit. Because ripening occurs very quickly and includes softening, ripe fruits are notoriously difficult to transport and store. Many fruits are picked green and subsequently treated with exogenous ethylene once they are ready to be consumed.
To prevent over ripening, ethylene can be absorbed by commercial products that react with and degrade ethylene.
Ethylene production accelerates senescence in cut flowers as well. Some powders that florists pack with their cut flowers contain compounds that degrade or interfere with ethylene production and so extend the life of the flowers.
Ethylene is synthesized from the amino acid Met by the sequential action of two hormones, ACC syntheses (which produces 1-amino carboxylic acid) and ACC oxidase. Because of the importance of controlling ripening commercially, several plants have been engineered to reduce expression of these enzymes and ethylene production. Ethylene is perceived by a small family of membrane-bound receptors found on the endoplasmic reticulum; as a gas, ethylene is freely permeable through the plasma membrane. When the receptors bind ethylene, they dissociate from and inactivate Constitutive Triple Response 1(CTR1), a negative regulator of ethylene signaling. In the absence of ethylene, CTR1 indirectly inactivates ethylene-responsive TFs, which, when CTR1 is inactivated, are free to initiate gene expression.
g. Hormone Action in Reproductive Development:
Several hormones influence the time at which a plant flowers, but their relative roles in this timing vary considerably. How hormonal signals affect flowering time is dependent upon environmental signals, particularly day length, and whether the plants flowers only once and then dies, as do annual plants, or continues to flower year after year, as do perennial plants like shrubs and trees. The molecular control of the decision to flower has been most thoroughly studied in short-lived annual plants, including Arabidopsis thaliana, pea, Maize, rice, and Lolium temulentum, a temperate grain plant. In most plants studied, the protein product of the FT gene is a mobile signal that moves from the leaves to the shoot meristem and initiates the expression of genes controlling reproductive development.
In some of these plants, the gibberellins augment this signal, whereas in others it has little to no effect except under unusual circumstances. Lolium seem to use gibberellins as the sole or primary signal to induce reproductive growth. In Arabidopsis grown in non inductive short days, gibberellins can promote flowering, whereas in perennial plants, gibberellin application either has no effect or restricts flowering. In bromeliads, including pineapple, flowering is strongly induced by ethylene, which is used commercially to synchronize flower and fruit production.
Most other hormones can influence the time of flowering indirectly through effects on growth rate and nutrient assimilation.
Hormones also participate in fruit development and ripening. A fruit is an enlarged ovary that contains the developing seed. Fruits assist in the successful propagation of the enclosed seed by providing an extra nutrient supply or by enticing consumption and thus dispersal by an animal. Pollination and seed development trigger auxin and gibberellin accumulation, which promote cell division and expansion in the ovary; these hormones have to be applied exogenously in the production of seedless fruit varieties. Commercially, many fruits are routinely sprayed with gibberellins to increase their size.